A semiconductor module for use in a power conversion apparatus such as an inverter includes a circuit, as shown in FIG. 13, formed by the combination of a switching element such as an IGBT (insulated gate bipolar transistor) 101 and a diode element 102.
To satisfy the need in recent years for reducing the size of such a semiconductor module, a semiconductor module formed by providing a diode element on a switching element has been proposed in, for instance, Japanese Unexamined Patent Application Publication No. 2000-164800 (paragraph 0019, FIG. 2).
However, when a diode element that includes silicon as a material (hereinafter called silicon diode element) is provided on a switching element that includes as a material conventionally and commonly used silicon (hereinafter called silicon switching element), both switching element and diode element need substantially the same electric current density. For this reason, the silicon diode element becomes large enough to substantially cover the whole of the silicon switching element, thus making it difficult to draw electric current out of the silicon switching element.
To this end, another semiconductor module is proposed in which a diode element that includes, as a material, silicon carbide capable of increasing the electric current density and reducing the size of the diode element (hereinafter called silicon carbide diode element), is provided on the silicon switching element.
For example, Japanese Unexamined Patent Application Publication No. 2004-95670 (paragraph 0079, FIG. 6) discloses a semiconductor module formed by directly providing a silicon carbide diode chip in the middle portion of a silicon semiconductor element or chip, which portion is on the emitter side of the element or chip.
Furthermore, Japanese Unexamined Patent Application Publication No. 2003-243612 (paragraphs 0019, and 0029; FIG. 1) discloses another module formed by providing on a silicon semiconductor switching chip a wide gap semiconductor, such as a the silicon carbide diode chip, having a greater energy band gap than that of silicon.
In the respective semiconductor modules as described above, an attempt to achieve a size reduction in semiconductor module is made by providing a silicon carbide diode element on a silicon switching element. However, the mere provision of the silicon carbide diode element on the silicon switching element does not provide a good thermal dissipation path for the silicon carbide diode element, thus causing heat produced by the diode element not to be dissipated but to be built up, which poses a problem of excessive temperature rise in the semiconductor module. The problem becomes significant in the case of an increased electric current density.